You probably need some trial and error until you find the right axis, this can be tricky sometimes.
If you can rotate your mesh in a 3d-program like SketchUp or Blender it might be easier to rotate it there and to use 0,0,0 for the rotation.
Some tips to find the right rotation:
try 0,0,0 first, and look how much rotation is necessary, in most cases 90° is enough.
try 90,0,0, then 0,0,0, then 0,0,90, don't rotate on two axises at once.

That's basically all about placing meshes.
The mesh will be attached to this three nodes, if they move or get ripped off the vehicle the mesh will move with them and keep it's offsets.